Interpretive Summary: Carotenoids are plant pigments present in all green plant tissue and can also be found in many plant tissues and organs serving as attractants for potential pollinators and seed dispersing herbivores. Carotenoids add important pigmentation to leaves and petals, as well as storage roots, as is found in carrot. Previous research has shown that these compounds are important in human health, as they serve as both vitamin A precursors as well as having antioxidant properties. Vegetables and fruits have been identified as good food sources of bioavailable provitamin A carotenoids for both human and animals. Carrot in particular is an important source of carotenoids in the human diet, providing up to 30% of pro-vitamin A in the United States alone. Biofortification, or biological enrichment, of pro-vitamin A carotenoid pigment content in food sources has been accomplished in tomatoes, carrots, and cereal crops such as rice, and knowing the mechanism of regulation of carotenoids will be important in understanding how these pigments can be incorporated in other plant species important in human nutrition. Using a method called real time quantitative PCR, the expression of nine genes within the carotenoid biosynthetic pathway was analyzed in carrots, and two genes associated with increased carotenoid accumulation in carrots were identified. Researchers studying gene expression, plant metabolism, and nutritional improvement or biofortification of crop plants will find this research of interest.

Technical Abstract:
Carotenoids are isoprenoid compounds synthesized in plants which serve as photoprotectants and provide plant tissues with red, orange and yellow pigmentation. These compounds are important in human health, as they serve as both vitamin A precursors as well as having antioxidant properties. Carrot provides an important source of carotenoids in the human diet, providing up to 30% of pro-vitamin A in the United States. While essential to human health, very little is currently understood about the accumulation of carotenoids in carrot. To better understand the molecular mechanism for carotenoid accumulation in carrot we used real time quantitative PCR to evaluate the expression of nine genes in the carotenoid biosynthetic pathway in storage root tissue. No significant difference was found among white, yellow, orange, and dark orange carrot roots in seven of the nine genes evaluated, but relative to yellow and white carrots, increased phytoene synthase 1 and phytoene synthase 2 expression was observed in orange and dark orange carrot roots. Increased phytoene synthase 1 and 2 expression was not observed in the leaf tissue of these genotypes, indicating a different mechanism for carotenoid accumulation in the leaf tissue of carrot. This study is the first to demonstrate that naturally-occurring mutations that dramatically increase carotenoid accumulation in orange carrot are associated with increased phytoene synthase 1 and 2 expression and it provides insights into the mechanism underlying the biosynthesis of these important nutrients.